1. Field
The following description relates to a wavelength-division-multiplexed transmission system, and more particularly, to a channel assignment technology for a wavelength-division-multiplexed transmission system.
2. Description of the Related Art
With the advent of Ethernet-based services such as Internet television services and user created contents (UCC), in recent times Internet traffic has increased considerably and widening of the network is required for such services.
A wavelength-division-multiplexed optical transmission system is regarded as the most efficient scheme to accommodate increases in traffic. In the wavelength-division-multiplexed optical transmission system, a number of wavelengths are multiplexed and transmitted through a single optical fiber. To improve the efficiency of the transmission, transmission rate per wavelength (channel) is increased and a variety of modulation schemes are employed based on high-speed channels.
Signals of 100 Gb/s per wavelength are used to satisfy bandwidth requirements at a given point, such as a high performance computer, a server, a data sensor, an enterprise network, and an Internet switching center, on which data traffic converges. To transmit signals of 100 Gb/s per wavelength, an optical transceiver employs new modulation schemes including phase shift key (PSK) modulation, by which a phase of an optical signal is modulated, and quaternary phase shift key (QPSK), by which two or more bits can be transmitted per symbol, instead of a simple non-return-to-zero (NRZ) scheme or a return-to-zero (RZ) scheme which modulates the amplitude of an optical signal between two levels.
However, in transmission of optical signals of different transmission rates and different to modulation formats through an optical path, significant cross-interference between the optical signals can take place, compared to the transmission of optical signals of the same transmission rate or the same modulation format. In this case, due to the nonlinear phenomenon such as cross-phase-modulation, then amplitude-modulated signals of NRZ or RZ format induce phase noise on phase-modulated signals of PSK or QPSK format, and thus the performance of the optical signal is deteriorated. In particular, since a large amount of data is transmitted at one time by means of the signal of 100 Gb/s per wavelength, the performance of the optical signal primarily needs to ensured.
To this end, conventionally, cumulative dispersion of an optical fiber is not completely compensated for, but several tens of ps/nm are left at each optical amplifier section, so that walk-off between optical signals can be controlled. Accordingly, it is possible to prevent nonlinear phenomena. This conventional method can be used in a point-to-point transmission system. However, since dispersion compensation should be conducted at each channel in a circulation or mesh net in which add/drop of an optical signal can occur at each optical amplifier section, a corresponding receiver is highly complex.
In another conventional channel assignment method, channels of low transmission rates are placed in a long wavelength band, and channels of high transmission rates are placed in a short wavelength band. However, since this method only minimizes performance deterioration by Raman crosstalk, it cannot suppress cross-phase modulation which is the main cause of performance deterioration of a phase-modulated signal, and only takes into account the transmission rate, and hence performance deterioration between channels due to the modulation formats cannot be prevented.